4) The word "cluetrain" is more or less constantly mentioned on Twitter: https://twitter.com/search?q=cluetrain
5) A search in Google books https://www.google.com/search?tbm=bks&q=cluetrain brings up more than 13,000 results, almost nineteen years after the original was published.
6) A search in Google Scholar https://scholar.google.com/scholar?en&q=cluetrain brings up more than 4,000 results.
7) A dig through old emails just turned up the earliest evidence (at least to me) of Cluetrain's inception: a draft of a joint JOHO (David Weinberger's email list) and EGR (Chris Locke's list) posting, vetted for input by yours truly. This was when the three of us were first sharing the co-thinkings that became Cluetrain in early 1999. That email is dated 30 October 1998, meaning that more than two decades have passed since this thing started.